Comparative analysis of nitrogen concentrations and sources within a coastal urban bayou watershed: A multi-tracer approach

Abstract

Fate and transport of nitrogen (N) in urban coastal watersheds continues to draw research interest due to serious impacts of N pollution and complexities with N sources and transport pathways. In this study, we used multiple tracers for source identification of N pollution (15N isotope in nitrate and chemical sewage tracers in water) and waters (using isotopes of 18O and 2H in water) in a coastal northwest Florida U.S.A. urban bayou fed by two contrasting streams, namely Jackson Creek traversing a dense residential area and Jones Creek flowing mainly through a wetland preserve. Results showed that the slightly higher δ15N-NO3− values in Jones Creek and the bayou were insufficient to distinguish N sources; yet the different chemical sewage tracer concentrations (e.g., sucralose, carbamazepine and sulfamethoxazole) clearly demonstrated the major N source from leaking septic tanks in the Jackson Creek sub-basin but not in the Jones Creek sub-basin. The higher concentrations of nitrate, which constituted over 98% of dissolved inorganic N in Jackson Creek, support active nitrification in sandy soils and steep terrain while higher δ15N-NO3− and much lower nitrate in Jones Creek are likely associated with denitrification in dense vegetative wetland and riparian zones. Episodic high nitrate concentrations and δ18O values in Jackson Creek preceded by periods of little rainfall indicated that the creek was sustained by subsurface flow with a steady input of nitrate. This study demonstrated the connection of land use and stormwater runoff generation to the forms of N entering urban waterways, the utility of N sourcing approaches, and the value of watershed-scale assessments for developing strategies to limit N loadings in urban settings.